Uplink channel estimation for nonorthogonal coded access

A promising multiple access scheme, which is called nonorthogonal coded access (NOCA), has been proposed for the fifth generation wireless communication systems to achieve higher service quality with limited radio resource provisions. However, uplink channel estimation in NOCA has not been well studied and the perfect channel knowledge has been assumed in the literature. In this paper, the uplink channel estimation for NOCA is investigated including the designs of pilot sequences and the channel estimation algorithms. Furthermore, two pilot patterns are proposed and evaluated in different spreading factors. The simulation results show that the proposed pattern designs enable accurate channel estimation using the pilots generated by nonorthogonal Zadoff-Chu sequences. In addition, a valuable observation is that the channel estimation is more accurate using smaller spreading factors in the case of high overloading.